1. Field of the Invention
The present invention relates generally to a method and apparatus for providing a positive pressure therapy particularly suited treat a patient suffering from congestive heart failure, and, more particularly, to a method and apparatus for providing a pressurized flow of breathing gas to an airway of a patient to treat Cheyne-Stokes respiration, sleep apnea, or other breathing disorders commonly associated with congestive heart failure.
2. Description of the Related Art
Congestive heart failure (CHF) patients commonly suffer from respiratory disorders, such as obstructive sleep apnea (OSA). Another such respiratory disorder CHF patients often experience during sleep is known as Cheyne-Stokes respiration. FIG. 1 illustrates a typical Cheyne-Stokes respiration (CSR) pattern 30, which is characterized by rhythmic waxing periods 32 and waning periods 34 of respiration, with regularly recurring periods of high respiratory drive (hyperpnea) 36 and low respiratory drive (hypopnea or apnea) 38. A typical Cheyne-Stokes cycle, generally indicated at 40 in FIG. 1, lasts about one minute and is characterized by a crescendo (arrow A), in which the peak respiratory flow of the patient increases over several breath cycles, and decrescendo (arrow B) variation in peak flow, in which the peak respiratory flow of the patient decreases over several breath cycles. The disruption in sleep, as well as the periodic desaturation of arterial oxygen (PaO2), stresses the cardio-vascular system and specifically the heart. Hyperpnea often causes arousals and, thus, degrades sleep quality.
Relatively recent developments in the treatment of sleep apnea includes the use of continuous positive airway pressure (CPAP), which is the application of a constant pressure to the airway of a patient. This type of positive airway pressure therapy has been applied not only to the treatment of breathing disorders, but also to the treatment of CHF. In using CPAP on a CHF patient, the effect of the CPAP is to raise the pressure in the chest cavity surrounding the heart, which allows cardiac output to increase.
Bi-level positive airway pressure therapy is a form of positive airway pressure therapy that has been advanced in the treatment of sleep apnea and other breathing and cardiac disorders. In a bi-level pressure support therapy, pressure is applied to the airway of a patient alternately at relatively higher and lower pressure levels so that the therapeutic pressure is alternately administered at a larger and smaller magnitude force. The higher and lower magnitude positive prescription pressure levels are known as IPAP (inspiratory positive airway pressure) and EPAP (expiratory positive airway pressure), and are synchronized with the patient's inspiratory cycle and expiratory cycle, respectively.
A publication entitled “Effects of Continuous Positive Airway Pressure on Cardiovascular Outcomes in Heart Failure Patients With and Without Cheyne-Stokes Respiration,” by Don D. Sin et al., which was published on Jul. 4, 2000 in Circulation, Vol. 102, pp. 61-66, describes how CPAP improves cardiac output in patients suffering from CHF and having both CSR and central sleep apnea (CSA), which is a cessation of breathing for a period of time not due to an obstruction of the airway. Additionally, it was found that CPAP can reduce the combined mortality-cardiac transplantation rate in patients with combined CSR-CSA who comply with CPAP therapy.
One approach to providing a pressure support therapy for the treatment of cardiac failure, CSR, or CSA is described in International Patent Application Publication No. WO 99/61088 to Resmed Limited (“the '088 publication”). According to the teachings of the '088 publication, a patient is provided with a ventilatory or pressure support using a blower and mask in much the same way as a conventional bi-level pressure support system. However, the system also derives an instantaneous ventilation of the patient by measuring the volume inspired, the volume expired, or half an average volume of the respiratory airflow over a short period of time. This derived measure of instantaneous ventilation is then used to adjust the level of ventilatory support in an attempt to reduce or eliminate short term changes in the derived measure of instantaneous ventilation. This is accomplished by comparing the derived measure of instantaneous ventilation with a target ventilation, which is a relatively long term measure, and controlling the level of pressure support based on the error between the two.
There are disadvantages associated with this method of providing pressure support to a patient to treat cardiac failure, CSR, or CSA. For example, in many situations, the average value of the past tidal volumes of the patient will not produce a target ventilation that, in turn, will result in sufficient treatment of the hypopneas and hyperpneas to counteract the occurrence of CSR. This is believed to be true because CSR has a continuum of severity and, depending on the level of severity, the target ventilation needs to be adjusted to values other than the average of the last 1-2 minutes. Moreover, the CHF patient may have some degree of obstruction that must be treated for its own sake, but also because these obstructive events appear to drive the CSR pattern as well. Therefore, a system that sets a target ventilation based on a long-term average of the past volumes does not address the interplay of obstructing airways and CSR. Using the instantaneous volume as the feedback variable requires yet another calculation, and this calculation is prone to errors due to small errors in the estimated patient flow and detecting the onset and termination of inspiration.
It is, therefore, desirable to provide a method and apparatus for treating OSA and CSR commonly found in the CHF population that adjusts the inspiratory and expiratory pressures to resolve the CSR and OSA based on detecting the peak flow where the effect of the error in the estimated patient flow is always smaller than that in the subsequent volume calculation. It is further desirable to detect the presence and severity of CSR and the level of pressure support presently intervening to treat the CSR more effectively than possible using conventional techniques.